Method for determining the nature of formations encountered in well drilling



Feb 15, 1944 D. SILVERMAN ET AL 2,341,745

METHOD EOE DETERMINING THE NATURE 0F FORMATIONS ENCOUNTERED IN WELL DRILLING Filed July 16, 1940 3 Sheets-Sheet l Feb 15, 1944- l D. slLvl-:RMAN E'r AL 2,341,745

METHOD FOR DETERMINING THE NATURE OF FORMATIONS ENCOUNTERED IN WELL DRILLING Filed July 1e, 1940 s sheets-sheet 2 Feb. 15, 1944. D. slLvERMAN ET AL 2,341,745

METHOD FOR DETERMINING THE NATURE OF FORMATIONS ENCOUNTERED IN WELL DRILLING Filed July 16, 1940 3 Sheets-Sheet 3 Qw/ J\/\ e lm r M d To. w J

A A y w 7 0 7 Hmmmmwwwmmhummmwwwmu H j Y M 9 3j Z 0 j M w M teamed ret'. is, iste unirse stares METHOD FOR DETEBMINING m NATURE F FDRMATIONS ENCOUNTERED WELL DEHLING Daniel Silverman and Robert W. Stuart, Tulsa, Okla., assignors to Stanolind Oil and Gas Company, Tulsa, Okla., a corporation of Delaware Application July 16, 1940, Serial No. 345,734

1 Claim.

This invention relates to the art of determining the nature of formations encountered during well drilling and apparatus therefor. More particularly it relates to a method of determining the amounts of substances, such as crude oil, salt water, limestone, gas, etc., contained in formations encountered during the drilling of a well by the rotary method, and still more specically to a method of achieving this result by observation of the properties of drilling iuids which contain cuttings of the formations penetrated.

Many methods have been devised for the purpose of obtaining some information as to the nature of the various formations traversed by a' well, such as the well-known electrical logging methods whereby such indications are obtained by means of appropriate electrical measurements. However, electrical logging methods have the disadvantage that they can be carried out readily only upon wells which have already been drilled and they are therefore not well adapted to give information as to the nature of formations penetrated by a drill bit during the actual drilling operation.

More recently a method of mud logging has been developed which consists of taking samples of drilling mud at regular intervals as it returns from the hole and testing these samples for minute amounts of crude oil and gas and/or determining their electrical conductivity, which is a measure of salt content. The method generally used for detecting crude oil in such samples is to subject them to irradiation with ultraviolet light and observe whether or not fluorescence is produced thereby. Y

Mud logging carried out in this way has the advantage over electrical logging in that it yields information as to the nature of the various strata penetrated by the drill, but it also has a number of disadvantages among which are that accurate determination of the amount of crude oil present is not made when that is the substance for which the test is made, and that there is necessarily a delay regardless of the test applied between the time that the particular mud sample was at the bottom of the well and the time of taking thejsample due to the fact that the mud requires a. considerable period, which is often of the order of one hour or more, to reach the surface. Obviously, any further drilling during this interval is done without the benefit of the test results, so that undesirable penetration of a water-bearing formation underlying an oil-bearing formation may occur, for example.

We have devised an improved method of mud logging whereby a major part of the time delay mentioned above is' eliminated together with most of the undesirable consequences thereof, and in addition the possibility of contamination is substantially eliminated. Broadly speaking,

our invention concerns the application of reverse circulation to mud logging and includes a number of modications which will be hereinafter fully described.

It is an object of our invention to provide a novel method of determining the nature of formations encountered during well drilling by means of tests applied to the drilling uid. Another object is to provide a. system for obtaining rapidly and efficiently information as to the nature of formations encountered during a rotary drilling operation. A still further object is to provide a method and apparatus whereby drilling fluid containing some of the formations being penetrated by a rotary drilling operation can be forced through the drill bit into the drill string and subsequently tested for at least one property indicative of the nature of a substance such as oil,'salt, limestone, gas, etc., which was present in that formation and whereby the results of such tests are made available in very short times. Other objects, uses and advantages of our invention will be apparent from the following detailed description read in conjunction with the draw ings in which:

Figure 1 illustrates schematically a vertical section through the earths crust and one form of apparatus for practicing our invention;

Figure 2 is likewise a schematic illustration similar to that of Figure 1 which shows apparatus for carrying out our invention according to another modification thereof;

Figure 3 illustrates one form of oil-detecting apparatus which can be used according to our invention shown in vertical-cross section;

Figure 4 is a horizontal cross-section along line 4 4 of Figure 3;

Figure 5 is a horizontal cross-section along line 5-5 of Figure 3;

Figure 6 is a partial cross-sectional view of another form of oil-detecting apparatus adapted to be used according to our invention wherein the indication of the amount of crude oil present in the surrounding fluid is recorded on a photosensitive medium; y

Figure '7 is a vertical cross-section of an apparatus for measuring the conductivity of drilling fluid within a drill string:

Figure 8 is a horizontal cross-section along line 8 8 of Figure 7 Figure 9 is a horizontal cross-section along line 9-9 of Figure 7; and

- Figure 10 illustrates diagrammatically the electrical apparatus and circuit used to record the indications obtained by means of the apparatus of Figure 7.

In one of its broadest aspects our invention comprises the determination of the nature of formations encountered during a. rotary drilling operation by causing the drilling fluid to pass through the drill bit from the bottom of the well into the drill string and subsequently testing at least a portion of this drilling fluid for at least one property indicative of the nature of a substance present in the formation being penetrated. In other words our invention comprises testing drilling fluid which has been forced into the drill string by reverse circulation to determine the amount of oil, salt, limestone and/or gas which was originally present in the formation being drilled.

Our invention is not only applicable to situationsin which a stratum containing one of these substances has already been penetrated but in the case of oil and gas, at least, our invention will enable the driller to ascertain that he is approaching a formation containing one or both of-them since small amounts of oil and gas are known to migrate and to exist in the formations immediately overlying strata containing large amounts of them. Thus, in drilling a well which approaches an oil-bearing formation, the oil content of the drilling fluid constantly increases due to the small amounts of oil present in that portion of the overlying formations carried out of the well in the form of cuttings.

As stated above, the periodic testing of drill mud returns is Well-known in the art but involves a disadvantageous delay in obtaining the test results. A considerable portion of this time delay can be eliminated according to one modification of our invention by employing reverse circulation in the entire rotary drilling operation and testing the mud returns for oil and/or other indicating substances. Because of the fact that there is a relatively large volume of drill mud contained in a well between the drill string and the casing or the hole itself and a relatively small volume, which is usually of the order of one-fifth, Within the drill string, the drilling mud will reach the surface in approximately one-fifth the time when using reverse circulation that it takes when the usual drilling procedure is employed, and consequently the returning mud can be tested and the results made available in that short time. For example, while a sample of mud may require an hour to reach the surface using ordinary circulation, by using reverse circulation, according to our invention the sample is taken and tested in less than fifteen minutes.

In many instances, however, it is not desirable to use reverse circulation during the entire drilling operation and we have therefore devised a modification of our invention which consists briefly in reversing the circulation of the drilling mud for a short time ywhen it is desired to test the nature of the formation being penetrated or to obtain an indication of the nature of a closely underlying stratum and testing the drilling n iud which is in the lower portion of the drill string either by lowering a suitable testing instrument or by taking a sample therefrom by means of a conventional bail and making the test at the surface of the earth. Our invention can best be understood in all its details by reference to the drawings.

Figurel illustrates, in highly simplified form, apparatus which can be used in carrying out that form of our invention in which reverse circulation of the drilling mud is employed continuously during the drilling operation. A well I is shown bottomed at II and having a casing I2 with a mud connection I3 below a pressure packer I4 which provides a seal between the casing and drill stem to give sufcient pressure for reverse circulation. Also shown within well I0 is a string of drill pipe I5 having a drill bit I8 at its lower end and a swivel Il at its upper end. During the drilling operation drilling mud is supplied from mud pit I8 by means of line I9, 20 and 2l to the mud connection i3, whence it passes down the annular space outside the drill pipe I5, picks up cuttings at the bottom II of the well and ascends through the usual opening in drill bit I6 and drill string I5 to swivel I1. The returning mud is then sent back to the mud pit after the conventional screening (not shown) by means of line 22, valve 23 and outlet 24. Prior to screening or any other operation, however, a portion of the returned mud passes through valve 25, testing unit 26 and valve 21 on its way to mud pit I8. As above stated, thev time required for mud to travel between the bottom Il of well I (l and mud testing unit 26 is relatively short. Testing unit 26 is adapted to determine continuously the presence and amount of crude oil, the electrical conductivity, the hydrogen ion concentration and/or the gas content of the drilling mud. Each of these tests have been used on a grab sample basis but we prefer to carry out all of these tests continuously and simultaneously.

While the particular tests themselves form no part of the present invention a brief description of the principles employed will be advantageous. The detection of crude oil has heretofore been carried out by subjecting samples of returned drilling fluid to irradiation with ultraviolet light and observing whether or not uorescence is produced. This fluorescence appears to be a property of the rather complex composition of the crude oil. This composition is changed by reilning operation so that the uorescence emitted by refined oils such as lubricating oils when irradiated by ultraviolet light is sufficiently different from the iiuorescence emitted by crude oil that there is no possibility of mistaking one for the other. Furthermore, the difference can be accentuated by the use of appropriate optical filters. This test therefore is a reliable one for the detection and measurement of small amounts of crude oil.

The electrical conductivity test can be carried out in a manner well-known to those skilled in the art, and as applied to drilling muds'is indicative of the presence of salt in the drilling mud and hence of salt or salt water if. the formation being drilled because the presence of salt very materially increaces the electrical conductivity of drilling mud. 'I'he hydrogen ion concentration or pH determination can also be carried out according to known principles and serves to indicate the presence of alkaline reacting materials, such as limestone. The determination of natural gas is usually carried out by passing a sample of gas removed from the mud under vacuum and mixing with a stream of air over a resistance maintained at a definite temperature by means of an electrical current and measuring any rise in temperature due to combustion of hydrocarbon gas by noting the increase in the value of the resistance.

In Figure l we have indicated that testing unit 26 operates on a continuous basis and we prefer that it should also record continuously all of the properties of the mud tested, but this feature is not essential to our invention since any of these tests may be applied periodically. However, the great advantage of continuously recording the test results, particularly when they are recorded on a strip showing the depth at which the mateassures rial in the mud being tested was originally present, is that an extremely useful log of the nature of the various formations traversed by the well being drilled is obtained.

In the second and usually preferred modification of our invention circulation of thedrilling fluid is maintained in the usual direction during most of the drilling operation. Figure 3, which illustrates this modification, shows a well 30 bottomed at 3i and having a casing 32 with a mud connection 33. Also shown is a string of drill pipe 3d having drill bit 35 at its lower end.

g the rotation of drill pipe 3d mud is supplied from mud pit 33 through line 31, pump 30, valve 33, line t and hose ti to a swivel (not shown) at the upper end of drill pipe 3Q. Thence the mud passes down drill pipe t@ through drill bit 35 to bottom 3l of well 3@ and up through the annular space around drill pipe 3d together with cuttings of the formation being drilled. The returned mud is withdrawn from the well through connection 33 and goes to mud pit 36 through line d2, Valve t3 and lines fili and t5. During all of this process, mud-testing unit te and supporting cable dll, which are also shown in Figure 2, are not present.

At desired intervals of depth or time, or when there is reason to suppose that the drill bit is in or approaching a stratum of interest either by reason of previous tests by the method being described or because the rate of drilling indicates that the bit is in a different formation, or becausea producing or other formation of interest is expected at a certain depth, the rotation of drill pipe 34 is stopped and the drilling mud around bit 35, which contains mat'erial from the formation composing the bottom 3| of well 30, is forced through bit 35 into the lower portion of drill string 3d. This is accomplished by closing valves 43 and 39 and opening valves 48 and 49 so that the direction of ow of the drill-y ing mud is reversed. After a very short time sumcient to cause a substantial amount of drilling fluid to enter drill string 34, mud pump 38 is stopped, the upper end of drill string 34 is broken and the mud in the lower end of drill string 34 is tested for one or more of the indicative properties above-mentioned.

One way of carrying out the testing operation is to lower a conventional wire line bail to obtain a sample of the fluid just above drill bit 35, withdraw it to the surface and test the sample by any of the procedures mentioned.

It is greatly preferred, however, to lower an instrument through drill string 34 which is capable of making the desired test, and this is particularly desirable when the test to be made is for the presence of crude oil or of the electrical conductivity. Figure 2 illustrates the position of testing unit 46 lowered on a cable 41 in carrying out this embodiment of our invention.

It will be apparent that this procedure has the advantage of being very rapid and of giving positive evidence oi.' the nature of the formation being drilled or of a formation being approached during the actual drilling operation, at which time this is most useful.

Any type of mud-testing instrument 48 which will give an indication of at least one property of the mud influenced by the nature of a substance present in the formation being drilled can be used. For the detection and measurement of crude oil, for instance, the oil detecting `unit which we prefer, is an ultraviolet irradiating unit and photoelectric pickup for indicating any resultant uorescence. Two modifications of this are shown in Figures 3 through 6. Referring to Figures 3, t and 5, a hollow steel casing 30 capable of withstanding high pressures is closed at its lower end by a heavy foot portion 5l which provides the weight necessary. tocause the unit to sink rapidly through the uids present in thedrill string, and at its upper end by a cap 53 having an axial opening 53 for the entrance of supporting and power supply cable 5t which corresponds to cable t1 of Figure 2. Cable 5t is iirmly attached to cap 52 by means of packing assembly 55 which also serves to make the unit uld-tight.

Casing 50 has at one side a window opening 55 completely closed by a window 51 made of quartz or other transparent material capable of transmitting ultraviolet as well as visible light, window 51 being held in position by means of molding t0 and screws 53. Within casing 5t a lightproof chamber 30 supported by brackets 3i contains a source of ultraviolet light 62 actuated by electrical current supplied from the top of the l well through cable 5t and leads 33. The source of ultraviolet light is preferably a lamp of the conventional mercury vapor type. Chamber 00 has at its lower end an opening tti for the emission of ultraviolet light across which filters 65 are mounted, these filters being capable of substantially completely removing both visible and infra-red rays.

The filtered ultraviolet lightl is then directed by suitable means through window 51 in order that it may irradiate fluid outside and in contact with this window. In the present design this is accomplished by means of a bent rod 38 made of quartz, a synthetic resin of the methacrylic type, or other material capable of conducting ultraviolet light axially therethrough. Also mounted within casing 50 by means of bracket 61 in a photosensitive cell 68 enclosed in a light-proof chamber 69 having on one side an opening 10 for the admission of light covered by lter 1l adapted to remove the ultraviolet rays therefrom Photosensitive cell 68 and filter 1| are arranged in such a way as to receive with a maximum of efliciency any fluorescent light produced by crude oil in the uid outside of window 51 under the influence of the ultraviolet light directed against it by rod 66. It has been found that the maximum fluorescent emission is obtained when the angle between the ultraviolet rays and the vertical axis of window 51 is of the order of 45, but this angle may vary between about 40 and about 50. Furthermore, maximum energization of the photosensitive cell is obtained when the included angle between the fluorescent rays reaching it and the ultraviolet rays as about 20 to about 40, preferably about 30.

Photosensitive cell 68 can be of any suitable type. For instance it can be of the photovoltaic type such as a copper oxide, silver or selenium cell which requires no auxiliary source of potential. In the apparatus shown in Figure 3 however, photosensitive cell 68 is of the photo-conductive type such as for instance a gas-lilled tube using a metallic rubidium electrode. The power to operate this cell is provided by means of leads 12, one f which contains a resistance 13. The variations of conductivity of cell 68 are measured by determining the potential difference across resistance 13, amplifying this potential difference by means of preampliiier 14 and conducting the amplifier output to the surface of the earth by means of shielded leads 15 which extend through cable 54 for recording in the usual manner.

Another modication employing a photosensitive medium such as a photographic lm is shown in Figure 6 which has the same type of ultraviolet light source as theunit'already described. ln this caser-a Vcarriera 80 is supported in the proper position to receive the uorescent light entering window 51. This light passes through lens 8| and lter 1| and is recorded on lm 82 which is driven from the lm magazine83 to the take-up magazine 84 by means of clockwork mechanism 85. For the sake of simplicity the detailed camera arrangement is not shown but will be obvious to one skilled in the art.

It is apparent that using either of these oil detecting units or an obvious modication thereof, the process described in connection with Figure 2 can be readily carried out as to the detection of minute amounts of crude oil and the measurement of relative quantities thereof in the drilling uid present in the lower end of drill string 34.

One type of unit for the determination of the electrical conductivity of drilling fluid in the lower end of drill string 34 is shown in Figures 7 and 8. This consists in a tubular body portion 90 of steel or other metal having a foot member 9| attached to its lower end but having a number of openings 92 therethrough for the ingress of drilling iluid. A cap member 93 completely closes the upper portion of body 90 and is supported by cable 94 which also contains suitable electrical conductors by means of packing gland 95. A tubular liner 96 of Bakelite or other electrical insulator is inserted within body portion 90, and within liner 96 two rings 91 and 98 of brass or other conducting metal are located, spaced from each other by tubular insulator 99, ring 91 being spaced from foot 9| by tubular insulator |00. Above ring 98 is another insulating spacer tube |01 and between the latter andA shoulder |02 of cap 93 is a disc |03 of insulating material, so that the interior of cap 93 is a chamber |04 which is kept free of mud. During the lowering of the conductivity testing unit, the mud in the drill pipe enters through ports 92 and leaves through ports formed in body 90 A and insulators 96 and |0I.

The electrodes for measuring variations in conductivity are the rings 91 and 98 already described. Between them two additional electrodes |06 and |01 are supported by means of insulating blocks |08 and |09, respectively, and this entire assembly is completely surrounded by a tube |||l of an insulating material which fits into appropriate openings in tube 99. In this way a separate cell is produced in which a/ standard sample of drilling mud can be placed without danger of contamination by the drilling mud entering the unit through ports 92 and which allows good heat transfer between the sample within and the drilling mud outside of tube ||0.

All of the electrode have electrical connections leading to the top of the well. Insulated conductor connects electrodes 91 and |06 and then extends through longitudinal groove ||2 in tube 96, opening ||3 and packing gland ||4 in insulating disc |03, and cable 94 to the surface of the earth. Electrodes 98 and |01 have a similar separate connections by means of conductors H5 and H6 extending through grooves Il1 and |8, openings l |9 and |20 and packing glands |2| and |22, respectively,

It has been found that in the determination of electrical conductivity of drilling muds, the effect of temperature is quite appreciable so that erroneous results will be obtained unless some method of compensating for the rather high temperatures prevalent in a well is provided. Consequently, a mud of known electrical conductivity is placed in the cell enclosed by insulator ||0 and the variation in conductivity between electrodes |06 and |01 provides a means for correcting the conductivity of the mud lling the entire unit including the space between electrodes 91 and 98.

Any suitable method of applying this correction can be used and we have illustrated one such method in Figure 10 which also shows the source of power and the recording device diagrammatically illustrated. For this purpose a source of alternating current is used which is indicated by a transformer |30, the output from which is applied across resistance |3| and electrodes 91 and 98 through leads and ||5 and across resistance |32 and electrodes |06 and |01 by means of leads and H6. It Will be seen that this system of resistances constitutes a Wheatstone bridge net work in which the source of potential is applied across one diagonal. The potential difference across the other diagonal ofthis bridge is applied to a conventional bridge rectifying system |33 and the rectified output is then sent to recorder |34. Obviously, by means of this apparatus variations in the relative resistivities between electrodes 91 and 98 and electrodes |06 and |01 can be obtained under any temperature conditions and a valuable record of the decreased resistance, i. e., increased conductivity of the drilling mud between electrodes 91 and 98 due to increased salt content, is obtained.

It will be readily seen from the above that we have provided a novel system of determining the nature of vformations encountered during a rotary Well drilling operation which provides precise information as to the presence of oil, salt, etc., in the formations being drilled, and does so in the most rapid fashion. Many other modifications will, of course, be apparent to those skilled in the art in the light of the above disclosure, and these are within the contemplation of our invention.

While We have described our invention in terms of certain specific embodiments thereof, we do not desire to be limited thereto but only by the scope of the appended claim.

We claim:

The method of determining the nature of a formation encountered during a well drilling op eration, wherein a circulating iluid is employed, which comprises interrupting said drillingv operation, forcing by reverse circulation a small amount of drilling fluid through the drill bit into the drill string above said bit, and testing, within said drill stem immediately above said drill bit. at least a portion of said small amount-of'drilling iluid for at least one property indicative of the nature of a substance present in the formation being penetrated by said drill bit.

DANIEL SILVERMAN. ROBERT W. STUART. 

